Pressure enhanced penetration with shaped charge perforators

ABSTRACT

A downhole tool, adapted to retain a shaped charge surrounded by a superatmospherically pressurized light gas, is employed in a method for perforating a casing and penetrating reservoir rock around a wellbore. Penetration of a shaped charge jet can be enhanced by at least 40% by imploding a liner in the high pressure, light gas atmosphere. The gas pressure helps confine the jet on the axis of penetration in the latter stages of formation. The light gas, such as helium or hydrogen, is employed to keep the gas density low enough so as not to inhibit liner collapse.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/085,635, filed May 15, 1998.

The United States Government has rights in this invention pursuant toContract No. W-7405-ENG-48 between the United States Department ofEnergy and the University of California for the operation of LawrenceLivermore National Laboratory.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the production of hydrocarbons from aborehole. More particularly, the invention relates to a method andapparatus for perforating and fracturing a formation surrounding aborehole.

2. Description of Related Art

Techniques for perforating and fracturing a formation surrounding aborehole are known in the art. The most common technique for perforatingand fracturing a formation to stimulate production includes the stepsof: 1) penetrating a production zone with a projectile, such as a shapedcharge; and 2) hydraulically pressurizing the borehole to expand orpropagate the fractures initiated by the shaped charge.

Modern shaped charges are widely used for both military and commercialapplications. Although the main operation is remarkably similar in bothapplications, there are at least two significant differences in thedevices actually employed. One difference is cost. Military applicationsgenerally demand much higher performance and, in particular, highreproducibility. This, in turn, requires the liner portion of the shapedcharge to be forged and precision machined.

In the commercial use of the shaped charge in oil or gas wellstimulation, the jet from the shaped charge is employed to create a flowpath from the reservoir to the wellbore. In this application, a largenumber of perforators is inserted into the wellbore in what is called agun. Although there are three basic types of guns, perhaps the mostcommon is the casing gun, which can be run into the well on a wirelineor conveyed by tubing. The charges are contained in a steel tube,protected from impact and from the well fluids, and are arranged so thatthey face radially outward from the vertical axis of the carrier. Inthese devices, the liners are pressed using powder metal technology andare relatively less expensive than those used in typical military uses,e.g., missile warheads.

Another factor that distinguishes commercial shaped charges from thoseused in weapons is standoff, i.e., the distance from the liner base tothe target (usually measured in charge diameters). The penetratingeffectiveness of a shaped charge jet is markedly enhanced by standoff.The reason is that shaped charge jets normally are formed with a highaxial velocity gradient, the tip moving at speeds of 6-10 km/s. Thestandoff distance allows the jet to stretch or elongate beforeencountering the target and, to first order, the depth of penetration isdirectly proportional to the length of the penetrator. There is anoptimum standoff. If the distance to the target is too great, thepenetration can be much less than if there were no standoff. This occursbecause the jet can only stretch a given amount before breaking; oncebroken the particles are easily deflected by small perturbations and nolonger produce a coherent, unidirectional penetrator. With optimalstandoff, typically 6-8 charge diameters (CD), the penetration can beenhanced by 50% or more, relative to that achieved with zero standoff.Commercial perforators, however, are rarely able to operate at more than1 CD because they must fit inside the casing gun which, in turn, mustfit inside the casing.

Techniques to increase the efficiency of hydrocarbon production in theborehole utilizing guns and pressurizing sections of the borehole havebeen described in the recent past. For example, Petitjean, U.S. Pat. No.5,355,802, describes a method and apparatus for firing a shaped chargethrough a gas zone of propellant combustion gases. A propellant isignited downhole, releasing gas into the borehole to pressurize aportion of the borehole. The firing of the shaped charges is delayeduntil the pressure level is significantly above the breakdown pressureof the formation, but still below that of the casing. Although such atechnique can improve the penetration effects of shaped chargeperforators, a need still exists to continually improve penetration of areservoir surrounding a borehole.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for perforatingand fracturing a formation surrounding a borehole and propagating thatfracture to increase the efficiency of hydrocarbon production in theborehole. The invention is advantageous because it benefits from theenergy of shaped charges to perforate and initiate fractures in theformation. In addition, it provides better propagation of the fractures.The greater efficiency is achieved by pressurizing an interior sectionof a sealable container of a downhole tool, such as a casing gun, usinga light gas, i.e., a gaseous substance which has a density less than airat the same conditions of temperature and pressure. The light gas isusually supplied (pressurized) and sealed at the earth's surface;although gas-generating materials which release the light gas within thegun prior to the firing of the shaped charges can also be employed. Thetravel of the shaped charge jet in the light gas atmosphere results in alonger, more narrow and stable jet—thus greater penetration.

According to one aspect of the invention, a casing gun, containingshaped charges surrounded by the pressurized light gas within the gun,is positioned in a production zone of a borehole. The shaped charges arefired and their liners collapsed within the light gas atmosphere. Theresulting shaped charge jet perforates the casing gunwall, penetratingthrough the wellbore fluids, through the well casing wall, into thereservoir rock and concomitantly the escaping light gas from within thegun increases the pressure level in the production zone. The pressurelevel in the production zone can be increased to significantly above thebreakdown pressure of the formation. To maximize the efficiency of thetechnique in a cased hole, the pressure level within the gun canapproach the maximum that can be applied to the wall of the gun and/orwell casing; however, penetration of a shaped charge jet has been shownby experiment to be enhanced by at least 40% (vs. air) by imploding aliner in the light gas atmosphere at pressures in the range from about1,500 psia to about 5,000 psia.

The fired shaped charges, creating the perforation tunnel through thewall of the casing gun and well casing, help to initiate fractures atparticular locations in the borehole. Thus, the shaped charges aredesigned to accomplish a dual purpose. First, the shaped chargesperforate the well casing. Second, after passing through the well casingthey continue their penetration into the formation sometimes initiatinga fracture. Such penetrations travel deeper than the procedures ofpreviously known techniques. Increased efficiency is achieved at theinitial penetration by increasing the jet length by squeezing on itsperiphery, which also produces a highly stabilized shaped charge jet.This is enabled by the firing of the shaped charges through thepressurized gas zone of substantially lower density within the guninstead of the higher density of conventional surrounding gases, such asair. The less dense, light gas zone permits effective collapse of theliner of the shaped charge.

The invention provides superior results to those obtained by the priorart because unlike the prior art, the pressure within the tool can bemaximized at the time the shaped charges are fired, thus providingincreased jet length and stability, and the shaped charge liners and jetcan function within a light gas atmosphere to improve jet penetration.Unlike techniques that release gas from a casing gun into the wellcasing outside the casing gun (via gas propellant materials) asdescribed by Petitjean, the present invention allows the shaped chargeliner to collapse against a less dense gas, thus initiating theformation of the shaped charge jet within the casing gun to creategreater jet length for extended penetration. Upon firing of the shapedcharges, the method of the invention provides increased perforation ofthe well casing and initiation of the fracture in a single step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the physical features and layout of a tool in accordancewith the present invention.

FIG. 2 illustrates a cross section of a shaped charge liner.

FIG. 3a shows calculations of jet formation and penetration in analuminum alloy target and FIG. 3b shows a snapshot of such calculationat 10 microsecs.

FIG. 4 illustrates a cross section of a simulated wellbore setup forconcrete penetration.

FIG. 5 shows the calculated penetration of shaped charges into aconcrete target.

FIGS. 6a, 6 b, 6 c and 6 d illustrate a shaped charge liner collapsewhen ambient air is 4350 psia.

FIG. 7 shows calculated penetration in a concrete target as a functionof helium pressure surrounding a shaped charge liner sealed within acontainer.

FIG. 8 illustrates a crossplot of the penetration data of FIG. 7 showingmaximum penetration in the range from about 1,500 psia to about 5,000psia surrounding the shaped charge.

FIGS. 9a, 9 b, 9 c, and 9 d illustrate four increasingly narrow andelongated shaped charged jets when four respective increasing heliumpressures (14.5, 1,450, 5,000, 10,000 psia) are exerted on a liner 10microseconds after initiation of explosion.

FIGS. 10a, 10 b, 10 c, and 10 d illustrate four increasingly stretchedshaped charged jets when four respective increasing helium pressures(14.5, 1,450, 5,000, 10,000 psia) are exerted on a liner 20 microsecondsafter initiation of explosion.

DETAILED DESCRIPTION OF THE INVENTION

The method of the invention is useful for enhancing the penetration of ashaped charge perforator into a reservoir material by imploding a linerof a shaped charge perforator in a high pressure, light gas atmospheretoward the reservoir material. High pressures are normally asuperatmospheric pressure greater than about 14.5 psia. The inventionincludes a gun apparatus adapted for positioning within a borehole in asubterranean earth formation. The apparatus includes means forcontaining the shaped charge within a chamber of a container such as acasing gun; means for surrounding the shaped charge with the pressurizedlight gas contained within the chamber; and means for firing the shapedcharge through at least a portion of a wall of the chamber toward theexterior of the chamber, through a well casing and eventually into thereservoir material.

The container is sealable. A light gas can be introduced within thecontainer to any predetermined pressure provided the gas is positionedto surround a liner of the shaped charge within the area in which theliner collapses during detonation and within the trajectory path of theshaped charge jet toward its target. The shaped charges are thus firedprecisely at an optimum predetermined pressure level causing them topenetrate deeper into the formation than they would otherwise. Thepressure provided by the light gas increases after the shaped chargeshave been fired. When fractures are formed, this pressure propagates thefractures further into the formation surrounding the borehole than theprior art techniques.

A feature of the invention is associated with firing the shaped chargesthrough the pressurized, light gas zone within the sealed container. Gasis supplied prior to firing of the shaped charges from any source,creating a light gas zone within the tool that surrounds the shapedcharge perforator. The light gas provides much less mass thanconventional pressurized gas as heavy as air, heavier than air, a liquidor a solid, which permits the shaped charges to penetrate deeper,helping to boost the efficiency of the fracturing and propagationtechnique. As a result, production from the well is increased.

FIG. 1 is a diagram illustrating the various components of a tool forperforating a well casing and propagating a fracture in a formationoutside the well casing. A tool 10 includes a sealable light gas loadingsystem and a perforation charge assembly. The perforation chargeassembly is responsible for firing shaped charges 12 which are mountedon a shaped charge holder and stabilizer bar 11 that is itself mountedto end caps 17 of an elongated tubular wall 15 of tool 10. Typicallyelongated tubular wall 15 can be any length and is usually from about 1to about 50 feet in length. The width or diameter of tube end caps 17can be any dimension fitting the tool within a well casing 18 whichseparates a borehole 20 from a reservoir 19. Fluid 25 may optionallyoccupy the space between tool 10 and well casing 18. The assemblyincludes a detonator device 13 connected to at least one oriented shapedcharge 12 via a firing cord 14. Detonator device 13 is activated by awire (not shown), to deliver an ignition signal on firing cord 14. As iscustomary in wireline tools, a cable 16 connects tool 10 to a surfaceapparatus including a sheave and winch (not shown) at the top of theborehole for delivering signals to and from tool 10 and for suspendingtool 10 in the borehole at a particular depth. Detonator 13 is connectedto wireline cable 16 via wire 21. Essentially no fast burning fusematerial need be employed along any of the wiring to cause detonation(firing) of the shaped charges.

The light gas loading system is responsible for pressurizing the volumewithin the tool. Although any means of sealing a light gas within cavity24 of tool 10 can be employed either by surface apparatus or downholepressurizing equipment, normally cavity 24 is filled and pressurizedwith the light gas at the surface through an orifice, such as opening22, and sealed by any of several means, such as plug 23. Since thethickness of elongated tubular wall 15 can be controlled to withstandany elevated pressure due to the light gas exerted from the interior orby wellbore fluids outside the tool, the invention allows the skilledartisan to utilize relatively light materials for tool walls and caps.

The operation of the invention can be described with reference to FIG.1. Initially, and usually at the surface, helium or other mixtures ofgases having densities less than air, are introduced into cavity 24 oftool 10 via opening 22 to achieve a desired pressure and then sealedwith valve 23. The tool is lowered into well casing 18 to a depth at thelevel of the targeted production zone of reservoir 19. The role of wire21 is to transmit a pulse to detonator 13 upon receiving an appropriatesignal from the operator of wireline cable 16. The pulse providesignition of firing cord 14, which in turn starts the firing of theshaped charges 12 within cavity 24 containing the pressurized light gas.The precise timing of the firing of shaped charges 12 is coordinated toretain light gas in the gas volume adjacent to the collapsing areabetween the liners. A jet created by the fired shaped charges penetratesthrough elongated tube wall 15 creating an opening through which thelight gas can also escape. The jet further travels through well casing18 and into reservoir 19. In addition, a head of fluid 25 above the toolposition is unnecessary to achieve maximum penetration of the shapedcharge jets through well casing 18 and into reservoir 19.

In one embodiment of the invention, the weight (and hence the cost) of acasing gun can be reduced if the pressure inside and outside of the gunwall is equalized. The penetration of a shaped charge perforator into atarget (e.g., concrete, reservoir rock, and the like) is little affectedwhen the (air) pressure surrounding the perforator is increased from 0.1to 10 MPa (14.5 to 1,450 psia). Since the gun must operate at the bottomof a well where the hydrostatic pressure can be tens of MPa, the gunwall thickness must be sufficient to withstand such hydrostaticpressures without imploding. Equalizing the pressure allows the wallthickness of the gun to be reduced substantially, and at least by afactor of 0.5, as compared to conventional gunwall thicknesses. Duringoperation of the gun, well fluids can not be allowed inside the gunbecause the high density of such fluids inhibits collapse of the linersof the shaped charges.

In an exemplary embodiment, the results of a computational study of theeffect of ambient pressure on shaped charge performance is described.Any conventional shaped charge can be employed in the invention. In theexemplary embodiment is used a single (commercial) perforator, i.e., anOMNI conical shaped charge (CSC) perforator, obtained from HalliburtonEnergy Services, Inc.

The (composite) liner is a mixture of primarily metal powders. Thecalculated jet tip velocity is compared with experimental data and thecalculated penetration is compared with measurements made in awell-characterized (6061-T6 aluminum alloy) target.

FIG. 2 illustrates a profile cutaway of the OMNI conical shaped charge(CSC) perforator. As indicated in FIG. 2, the outer base diameter D ofthe steel tamper 44 is approximately 46 mm. The explosive charge 42weighs approximately 22.7 g and consists of about 98.5-99% RDX, with theremainder a wax filler. The liner 40 consists of a mixture of tungsten(45.20%, by weight), tin (11.05%), copper (43.19%), and graphite (0.53%)powders, together with a small (0.03%) amount of lubricating oil. Thecalculated density of the fully compacted liner is approximately 11.19g/cm³. Measurement of the actual density, using the method ofArchimedes, yields a value of approximately 10.15 g/cm³ [M. G. Vigil,Conical Shaped Charge Pressed Powder, Metal Liner Jet Characterizationand Penetration in Aluminum, Sandia Report, SAND97-1173, May 1997.] sothat an initial gas porosity of 0.0929 can be inferred. (A Grüneisenequation of state for the fully compacted powder is derived applying theresultant parameters: c₀=3.79 km/s, s=1.592, g₀=1.8, and b=0.5. Here, c₀is the bulk sound speed, s is the slope of the shock Hugoniot (in shockvelocity-particle velocity space), g₀ is the initial Grütneisenparameter, and b is the first order volume correction to g₀.).

All simulations are performed with the CALE hydrocode, developed at LLNLby R. Tipton [See LLNL Laboratory Report 961101, November 1996]. Thepore compaction treatment in this code follows closely the standardp-alpha formulation initially devised by Carroll and Holt [See M. M.Carroll and A. C. Holt, J. Appl. Phys., 43, 759-761 (1972)]. A Hugoniotelastic limit of 50 MPa is prescribed, with complete pore crushupoccurring at 161 MPa. No independent measurements are made of the linerstrength so that, in effect, the strength model constituted a degree offreedom available to help fit the penetration data. The standardSteinberg-Guinan ductile failure model available in CALE, employed withparameters derived for copper, results in excellent agreement betweenpredicted and measured jet tip velocity and in depth of penetration in(6061-T6) aluminum alloy targets (the experiments are described by M. G.Vigil above).

FIG. 3a shows the calculated penetration as a function of time, togetherwith FIG. 3b, a snapshot crossection of such penetration into 6061-T6aluminum target 52 at approximately 10 microseconds. The velocity of thecalculated jet tip 50 at this time is approximately 6.4 km/s, the samevalue measured from the radiographs in the experiment. The finalpenetration is approximately 265 mm, again in excellent agreement withthe interpolated curve derived from the measurements (the calculation isperformed at a standoff of approximately 22.1 mm; the experiments areperformed at standoffs of approximately 6.35, 152.4, and 482.6 mm). Thestandoff position chosen for the calculations is the same as theposition of the first target plate employed in the concrete penetrationexamples described hereinafter.

Although concrete is not a perfect surrogate for reservoir rock, itprovides a suitable comparison for predicted penetration of jets fromshaped charges for utilizing data from experiments in which the ambientpressure is atmospheric or otherwise. Several calculations of thepenetration of shaped charges into standard (API RP43) concrete targetscan be performed when the pressure surrounding the perforator is varied.

FIG. 4 illustrates the downhole setup 30 (simulating area about awellhead such as that described by shaped charge 12, elongated tubularwall 15, well casing wall 18 and reservoir rock 19 of FIG. 1) from whichconcrete penetration is measured so as to essentially replicate the APISection 1 target. The outer boundary 32 of the setup is rigid. The firststeel target plate 34 represents a gun wall and the second steel targetplate 36 adjacent the concrete 38 represents a well casing. P₁ is theambient pressure surrounding the perforator (i.e., the pressure withinthe gunwall), P₂ is the pressure in the wellbore (i.e., the wellborepressure which is located outside the gunwall and inside the well casingwall), and P₃ represents the reservoir pressure having a 4 inch diameterD2.

The concrete model employed is consistent with the specification for APIRP43 Section 1 targets and fits the shock Hugoniot data reported forthis material by Furnish [M. Furnish, Shock Properties of the API-43Concrete and Castlegate Sandstone (ACTI Near Wellbore MechanicsProject), Sandia National Laboratory Draft Technical Memorandum, 1997.].The initial gas porosity is approximately 0.18, corresponding to adensity of approximately 2.15 g/cm³. The unconfined compressive strengthis approximately 51.7 MPa (7,260 psi), and the strength is increasedwith pressure up to a maximum of 160 MPa at a pressure of 1 GPa.

Penetration calculations with air as the surrounding gas are initiallyperformed. FIG. 5 shows the results when the pressure P₁ (air) is variedin a production zone 54 from 0.1 to 20 MPa (i.e., 14.5 to 2,900 psia).The reservoir and wellbore pressures (i.e., P₂ and P₃, respectively) areassumed equal and set to 1,450 psia (for consistency with industrypractice, English units are used). It is observed that the penetrationdecreases monotonically with increasing ambient pressure (simulatedinterior gun pressure surrounding the perforator), but that the finalpenetration is only about 8% less as P₁ increases from 0.01 to 10 MPa.The calculation with P₁ set to 14.5 psia is in reasonably good agreementwith experimental data. As shown in FIG. 5, the average measuredpenetration in the present setup is 19.7 inches when the manufacturingprocess is under control. The measured range of penetration inproduction zone 54 is from 14 to 22 inches during a production run. Asthe simulated interior gun pressure containing air exceeds 1,450 psia,the penetration is seen to rapidly diminish.

FIGS. 6a, 6 b, 6 c and 6 d illustrate stages of shaped charge linercollapse when P₁ (air) is increased from 2,900 to 4,350 psia. FIG. 6ashows the intact, original liner prior to detonation, i.e., at t=0, andperetration through toolwall 15, well casing 18 and into reservior rock19. As illustrated in FIG. 6b at 10 microseconds, when the jet at lowambient air pressure is already well developed, no jet is observed; theliner collapse has been inhibited by the formation of a high-pressureair bubble. At 40 microseconds (illustrated in FIG. 6c), when thepenetration in the concrete at low ambient air pressure is over 5 in.,an annular jet has formed, and only the first steel plate has beenperforated. As illustrated in FIG. 6d at 200 microseconds, the jet hascompletely broken up and even the steel casing has not been completelyperforated.

It is evident that the pressure exerted on the jet is beneficial sincethe jet is confined on the axis during the latter phase of formation.However, as increasing pressures are exerted within the gun, theincreasing mass of the air inside the conical liner becomes increasinglydifficult to expel during detonation as the density of such gases isincreased. Eventually, the density effect inhibits the formation of astable jet altogether, as seen in FIG. 6d.

In the method of the invention a light gas, i.e., lighter than air, suchas hydrogen or helium, is employed to surround the shaped charge insidethe gun. At the same pressure and temperature, the density of the lightgas within the gun is less by a factor of more than about 14 with theformer and more than about 7 with the latter. The practical advantage ofusing an inert gas outweighs the theoretical advantage of utilizing areactive gas such as hydrogen.

FIG. 7 summarizes the results of varying helium P₁ from 14.5 to about10,000 psia. In this case, increasing the helium ambient pressure, P₁,from 14.5 to 1450 psia substantially increases the penetration. Forinstance, results at t=800 microseconds indicate a penetration of about26 in. at 1450 psia in the helium system of FIG. 7 compared to apenetration of about 19 in. at the respective conditions in thepressurized air system of FIG. 5, i.e., a increase by a factor of morethan 1.3. Increasing P₁ by another factor of at least 3, to, for example5,000 psia, further increases the penetration. At still higher pressure,the penetration begins to decrease; when the initial surrounding heliumpressure is 10,000 psia, the penetration is still slightly higher thanthat achieved with air at the increased pressure of 1,450 psia; the gasdensity is about the same in the latter two cases.

For the present design, the maximum penetration, with helium, occurswhen P₁ is between about 1,500 and about 5,000 psia. FIG. 8 crossplotsthe data in FIG. 7. It is observed that, although final penetration hasnot stabilized in all the calculations, the penetration of the shapedcharge surrounded with helium at 1,500 psia is at least 25% greater thanthe penetration obtained when the shaped charge liner is surrounded byair at normal pressure.

FIGS. 9a-9 d and 10 a-10 d illustrate the physical basis for thisincreased performance. In both figures, cross sections are overlaid ofonly the liner material for each of the calculations described in FIGS.7 and 8. In FIG. 9a-9 d, the overlays are displayed at 10 microseconds,when the jet tip velocity has attained its maximum value, prior toperforation of the first plate (gun wall). The base profile 75 is theliner cross section in the 14.5 psia calculation, i.e., FIG. 9a, havinga jet tip base 80 and a jet tip 81; the jet overlays 76, 77, and 78 ofFIGS. 9b-9 d, respectively, are the result of the other calculations. Itis clearly observed that, as the initial helium pressure surrounding theliner is increased from 1,450 psia to 5,000 psia, to 10,000 psia, inFIGS. 9b-9 d, respectively, the base 80 of the jets is forced to recedeand an increasingly narrow and elongated jet, including jet tip 81, isproduced.

FIGS. 10a, 10 b, 10 c and 10 d depict the liner profiles at 20microseconds. As the initial surrounding pressure increases from thebase profile of jet 88 of FIG. 10a, the jets 89, 90 and 91 of FIGS.10b-10 d, respectively, are seen to elongate and their cross sectionsdiminish. When P₁ is 10,000 psia in FIG. 10d, the jet tip 92 is stillslightly ahead of the low-pressure case, but the calculation showsevidence of jet breakup beginning to occur. The interface treatmentimplicitly produces the breakup effect when the cross section getssufficiently small; gas and jet material are then intermixed, and thelocal density is concomitantly reduced which, in turn, tends to decreasepenetration.

Accordingly, analysis of a shaped charge perforator (jet) has shown thatthe penetration can be substantially enhanced by imploding the liner ina high pressure, light gas atmosphere. With the light gas heliumpressurized at about 1,500 to about 5,000 psia, the penetration intoconfined concrete cylinders is increased by at least 40% in comparisonto that achieved when the liner of a shaped charge is surrounded by andoperated in air at standard temperature and pressure. The increasedperformance results from the gas pressure acting to confine the jet onthe axis of penetration in the latter stages of formation. Since highdensity is concomitant with high pressure, a light gas, such as heliumor hydrogen, allows the gas density to be kept low enough to not inhibitliner collapse.

Commercial perforators used in oil or gas well stimulation are normallydisadvantaged by the short standoff forced upon them by their insertionin casing guns; inadequate space is available for the jets to stretch tooptimum length for use in casing guns. Thus, high gas pressure cancompensate for the lack of space by squeezing on the periphery of thejet, thereby producing added elongation. In the downhole location, thehigh pressure and lighter gas surrounding the shaped charge perforatorhas the added advantage that the wall thickness of the casing gun can bediminished, since the pressure differential between the internal gun andwellbore is thereby decreased. Such a small or essentially nonexistentpressure differential has the potential for producing a lighter, andhence less costly, gun assembly. Balanced against these potentialadvantages is the added complexity in positioning a high pressure systemin the downhole location.

The inventive system can utilize a pressure regulator that senses theexterior wellbore pressure and automatically adjusts the interior gunpressure to minimize the pressure differential.

The invention is further illustrated by the following example which isillustrative of specific modes of practicing the invention and is notintended to be exhaustive or to limit the invention to the precise formsdescribed.

EXAMPLE

Shaped charges are fired at two (2) API Section 1 targets, each using4⅝″ (OMNI) guns (12 SPF). Both concrete targets have been poured on thesame day and cured for the same period. In one target the gun isoperated with interior ambient air pressure and in the other a sealed2,000 psi (138 bar) helium pressurization system is employed.

Using the 4⅝″ OMNI gun apparatus having means for maintaining thepressurized helium downhole, the average penetration from 37perforations is increased 40.3% over that obtained with the conventionalperforating apparatus and system (the standard deviation being 11.3% ofthe mean for the pressurized helium system and 12.9% for theconventional system).

It should be noted that such results exceed the predicted performance.(The predicted simulations are made for an ideal (axisymmetric)perforator.) The presence of the high-pressure light gas surrounding thejet inhibits instabilities that enable the jet to wander off axis andmarkedly decrease penetration. The resulting perforations in the gunsurrounded by helium are visually much straighter and cleaner and thegun-holes appear much more uniformly round. Such penetration resultsalso suggest enhanced hydraulic fracturing can be accomplished with thepressurized light gas (helium) system.

Although particular embodiments of the present invention have beendescribed and illustrated, such is not intended to limit the invention.Modifications and changes will no doubt become apparent to those skilledin the art, and it is intended that the invention only be limited by thescope of the appended claims.

The invention claimed is:
 1. A method for enhancing the penetration of ashaped charge perforator, said method comprising: imploding a liner ofsaid shaped charge perforator in a high pressure, light gas atmosphere.2. The method of claim 1 wherein said light gas atmosphere comprises agas or a mixture of gases having a density less than air at the sameconditions.
 3. The method of claim 2 wherein said light gas compriseshelium or hydrogen.
 4. The method of claim 1 wherein said light gascomprises helium.
 5. The method of claim 1 wherein said high pressure isabout 500 psia to about 10,000 psia.
 6. The method of claim 5 whereinsaid high pressure is about 1,000 psia to about 6,000 psia.
 7. Themethod of claim 1 wherein said high pressure, light gas atmosphere andsaid liner is located within a sealed casing gun.